Agriculture Reference
In-Depth Information
region, urediniospores can be found at almost
any time of year, especially west of the Cascade
Mountains where the mild winters and cool
summers are favorable to stripe rust survival. The
cool night temperatures and dry conditions during
summer in the major wheat growing areas east of
the Cascade Mountains allow urediniospores to
retain viability for extended periods. Uredinio-
spores that are produced in late summer and early
fall from spring wheat fi elds, volunteer plants,
and grasses infect the winter wheat crops. The
fungus overwinters in plant tissue as mycelia
and often as viable urediniospores on plants
(Hungerford 1923). According to Rapilly (1979),
stripe rust can survive temperatures as low as −
10 ºC. Snow cover provides favorable conditions
for both the wheat plants and stripe rust to survive
the winter. Heavy local survival of the stripe rust
pathogen over the winter will lead to early
epidemic development in the spring. Stripe rust
forecasting models based on December and
January temperatures have been successfully used
in the Pacifi c Northwest to predict stripe rust
epidemics (Coakley et al., 1982, 1984; Line 2002;
Chen 2005).
Stripe rust can also overwinter in regions with
cold winters. Stripe rust can be endemic in the
Gallatin Valley and Flathead Lake area in Montana
(Sharp and Heln 1963) where the fungus has
overwintered. In 2006, stripe rust severity was
unusually high in Alberta but light in the US
Pacifi c Northwest, indicating winter survival of
the rust in Alberta. In North Dakota and Minne-
sota stripe rust generally does not overwinter.
However, in 2006, stripe rust was observed at St.
Paul, Minnesota, on April 26, a month earlier
than the fi rst observation of the disease at Pullman,
Washington, and also much earlier than reports
of the disease in Missouri, Illinois, and Indiana,
which indicated that the pathogen survived the
2005-2006 winter in Minnesota. The cold winters
and hot summers in the northern Great Plains
region usually limits stripe rust development in
this region.
In California stripe rust regularly overwinters
and survives the summer on wheat crops
and grasses at high elevations along the coast
(Tollenaar and Houston 1966), and on wheat
grown as forage or cover crops. Survival of stripe
rust on volunteer wheat plants in irrigated fi elds
resulted in a widespread epidemic in California in
1974 (Line 1976). Northeastern California, where
stripe rust oversummers, may be a source of inocu-
lum for southern Oregon, southern Idaho, and
northern Utah, as well as for central California.
Stripe rust inoculum is exchanged among Califor-
nia, Arizona, New Mexico, and northwestern
Mexico.
The stripe rust pathogen generally does
not survive the summers in the Great Plains,
Mississippi Valley, and the southeastern states
due to extended hot and humid conditions and the
long period between harvest and planting of wheat
crops. The late-planted wheat crops at high eleva-
tions in Mexico may provide initial inoculum for
infection in the fall in the southern Great Plains
(Line 2002). Wheat crops in the Rocky Mountain
areas from Colorado to western Texas may con-
tribute stripe rust inoculum to the Great Plains.
Stripe rust infections occur soon after emergence
of wheat in the late fall and early winter in Texas,
Louisiana, Arkansas, and Mississippi. If stripe
rust occurs, it usually develops slowly in the winter
and at a faster rate in the early spring, providing
inoculum for areas further north and east. The
scope and severity of epidemics depend on inocu-
lum in the south, on wind directions, and on tem-
perature and moisture conditions in the region
east of the Rocky Mountains. Stripe rust has been
reported in New York, but has never caused sig-
nifi cant damage north of Ohio and Virginia.
Origin and historical importance
Stripe rust of wheat is a long established disease
in Asia and Europe (Stubbs 1985). Stripe rust was
fi rst recognized in the US in 1915 in Arizona
(Carleton 1915). However, examination of her-
barium specimens has indicated that the disease
was present in the western US before 1892
(Humphrey et al., 1924) and possibly occurred in
California in the 1700s (Smith 1961). The
Caucasus region is the presumed origin of stripe
rust (Stubbs 1985; Line 2002).
From 1957 to 2005, the US experienced four
waves of regional epidemics of stripe rust. The